Conventionally, different kinds of communication systems are used to provide voice, data, and video communication services to subscribers. For example, a telephone system is conventionally used to provide voice services, a circuit-switched data system or a switched packet network is used to provide data services, and a cable television (CATV) network is used to provide motion video transmission services. These systems exist side-by-side, and each has its own specific design requirements and limitations (for example, in terms of transmission encoding format and modulation, available bandwidth, and signaling protocol), that make each uniquely suited for providing one service, but generally poorly suited for providing a different service. For example, the persubscriber end-to-end bandwidth of the telephone network is too narrow to provide a broad range of data communications efficiently, and it is wholly insufficient for providing motion video services. And a CATV network is ill-suited for providing two-way communications, particularly interactive services. Hence, the various systems exist side-by-side, and each typically provides either only one service or a very limited spectrum of services.
Such proliferation of many parallel facilities each having only limited use is obviously very inefficient. What's more, even though certain of these systems are very new, future services are already being envisioned which exceed the intrinsic capabilities of these systems and hence will require the installation of yet other communication systems. Clearly, what is needed is a single system that is capable of providing universal information services: all current communication services, as well as new communication services that may foreseeably be desired in the future.
Significant effort is being devoted internationally to define a system that would provide in a single digital network at least some plurality of voice and data services that have heretofore been provided by separate systems. A system of this nature is generally referred to as an integrated services digital network (ISDN). For example, the International Consultative Committee on Telephony and Telegraphy (CCITT) is carrying on the work of trying to develop a set of international standards for an ISDN. However, most of this effort is devoted to specifying the form that communications being transported to or from a subscriber should take, and the communication protocol under which communication paths to and from a subscriber should be established. But a design of a communication network that supports the communication protocol, implements the communication format, and actually provides envisioned voice, data, and video services in integrated form, is lacking in the art.